Insist on Cython prior to 3.0

[drbsg]

This is a back-port of pull request #1061 to the 5.3 maintenance branch.

[corranwebster]

PR looks good, not sure what's happening with CI

[jwiggins]

Looks like the Wx environment needs attrdict installed before installing Wx

[drbsg]

Looks like the Wx environment needs attrdict installed before installing Wx

I am not familiar with EDM, so would adding this dependency to dependencies for both versions in ci/edmtool.py be a good way to achieve this?

[corranwebster]

I think the right way to fix would either be to restrict the WxPython version or to add attrdict here: https://github.com/enthought/enable/blob/84b611dcc329c7558f5512b861fb8e002f90f39f/enable/init.py#L52

The problem seems to be this version of WxPython not declaring all dependencies.